In general, in the case of an electromagnetic field that is radiated to free space from an antenna, a reactive electric electromagnetic field is greater than a radiated electromagnetic field within a predetermined region, and the predetermined region is referred to as a reactive near-field region. Then, the radiated electromagnetic field becomes greater than the reactive electromagnetic field in a region that is referred to as a radiating near-field region. Here, the radiated near-field region is also referred to as the Fresnel region, and an electromagnetic field existing in this region is referred to as the Fresnel electromagnetic field.
Radio waves radiated from the antenna are transformed almost to plane waves after passing through the Fresnel region. At this time, a region where a distance R of the radio waves radiated from the antenna is greater than 2L2/λ is referred to as a far-field region or the Fraunhofer region, and an electromagnetic field that exists in this region is referred to as a far-field. Here, L denotes an antenna aperture length, and λ denotes an operation frequency wavelength.
A radio device such as a mobile communication base station radiates high power electromagnetic waves. Therefore, the strength of electromagnetic field should be known and a safe distance should be guided.
In order to obtain the Fresnel field or the far-field of the finite distance, measurement can be directly carried out at the corresponding location or in an anti-reflection chamber. Here, the anti-reflection chamber may be called an anechoic chamber.
When locating an antenna under test (AUT) for obtaining an electromagnetic field of a specific distance such as the Fresnel field or the far-field, the measurement can be directly carried out at the corresponding location if the anti-reflection chamber is large enough to include a distance to be measured. However, when the distance to be measured is larger than the size of the anti-reflection chamber, an anti-reflection chamber that is larger than the existing anti-reflection chamber should be provided. In addition, a distance between a probe and the AUT should be variable, and therefore the anti-reflection chamber should have distance control equipment.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.